156 



Science of Plant Life 



I branches or leaves. They re- 

 spond to contact with a sup- 

 port by coiling tightly about it. 

 After attaching themselves, 

 they develop mechanical tissue 

 which gives the plant a firmer 

 support. In some vines, like 

 the Boston ivy, the tendrils 

 have at their tips sensitive disks 

 which become cemented to the 

 support. This type of tendril is 

 especially effective in taking 

 hold of the bark of trees, rock 

 cliffs, and walls (Fig. 87). 



In the tropics, climbing stems 

 may attain a length of more 

 than looofeet. Thus the water 

 transpired by the terminal 

 leaves has to be carried for 

 about a fifth of a mile within 

 the plant. This suggests the 



need of an efficient conductive system in a climbing plant, and 

 explains why the bulk of its slender stem is made up of con- 

 ductive tissue. 



Stems of hydrophytes. We have seen that submerged 

 leaves have distinctive forms and characteristic internal 

 structures. Under- water stems also differ from those of 

 land plants. In floating plants like duckweed, water hya- 

 cinth, and Salvinia, the stems are short. Their conductive 

 systems are poorly developed, and they are practically without 

 mechanical tissue. 



FIG. 88. Climbing stems on a tree 

 trunk in a tropical forest. 



